Seismic surveys image or map a subterranean formation by imparting acoustic energy into the ground and recording reflected energy or “echoes” that return from subsurface rock layers below. Some conventional sources of the acoustic energy include explosions, air guns, vibrators, and typically positioned on or near surfaces of the earth. Each time the acoustic energy source is activated, a seismic signal is generated. The seismic signal travels into the earth, where it is partially reflected, and, upon its return, may be detected at many locations on the surface as a function of travel time. Sensors commonly used to detect returning seismic energy include, but are not limited to, geophones, accelerometers, and hydrophones. The returning seismic energy is recorded as a continuous signal containing information related to displacement, velocity, acceleration, and/or other recorded variation as a function of time. Multiple combinations of acoustic energy source and sensor can be subsequently combined to create a near continuous image of the subsurface that lies beneath a survey area. One or more sets of seismic signals may be assembled in a final seismic survey.
Time-lapse methods has been used to improve reservoir management around the world in many hydrocarbon producing basins. Time-lapse seismic survey involves acquiring, processing, and interpreting repeated seismic surveys over a producing hydrocarbon reservoir. A four-dimensional (4D) seismic dataset may include a set of three-dimensional (3D) seismic data taken at different time points. Changes occurring in the reservoir can be determined by comparing repeated datasets (i.e., datasets collected over the same reservoir at different time points). Thus, time-lapse methods can monitor production-related changes in the reservoir.
In order for time-lapse seismic survey to be effective, changes in the reservoir properties must cause a detectable change in seismic signal. Saturation and/or pressure changes of a hydrocarbon producing reservoir can create a difference in elastic properties (e.g., velocities and density) that may be large enough to be detected by surface seismic surveys. Strength of the signal will depend on a number of factors including, but not limited to, magnitude of change in the reservoir properties and sensitivity of the elastic properties to these changes.
Hydraulic fracturing is an economically important technology applied to oil and gas reservoirs to increase oil and gas production. During hydraulic fracturing, highly pressurized fluids are injected into reservoir rock. The pressurized fluids overcome the breaking strength of the rock and induce fractures that act as pathways by which oil and natural gas can migrate to the borehole and be brought to the surface. Mapping and characterizing these fracture systems may be important in order to more fully realize the economic benefits of hydraulic fracturing. Despite advances in time-lapse seismic methods, fractures created by hydraulic fracturing can still be difficult to detect using time-lapse seismic methods.